In general, low specific speed centrifugal pumps operating at moderately high speeds (20,000-30,000 rpm) are well-suited for use in gas turbine engines fuel systems. However, prominent drawbacks associated with the utilization of such pumps are the inefficiency at reduced (off design) flows and the limitations imposed upon fuel temperatures and the amount of vapor contained in the fuel at inlet pressure.
The reduced or off-design flows in centrifugal pumps beget inefficient operation owing to the parasitic losses caused by the fuel drag occurring between the impeller and the casing and flow recirculation. The provision of freewheeling discs (which define a shroud) between the impeller and the casing to reduce fluid drag is described in U.S. Pat. Nos. 803,759, 921,118 and 1,032,828, French Pat. No. 346,865 and Austrian Pat. No. 19509.
However, the incorporation of a freewheeling disc between the impeller and the casing presents certain difficulties. First, rotation of the discs will be impeded by mechanical friction and the action of recirculating fluid, thereby resulting in less than optimum disc speed. Second, the lack of a positive drive could result in a drastically reduced speed or stoppage of the discs from fuel contamination or other foreign particles.